It has long been known to obtain color images by contact reaction between an electron-donating or protonaccepting colorless organic compound (hereinafter referred to as a color former) and an electron-accepting or protondonating solid acid (hereinafter referred to as a color developer). Examples of specifically utilizing this phenomenon are pressure-sensitive copying (or recording) papers (for example, U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, 3,418,250, and 3,672,935), heat-sensitive recording papers (for example, Japanese Patent Publication Nos. 4160/68, 7600/68, 14039/70, U.S. Pat. No. 2,939,009, etc.), and so forth.
Further, a printing method is known which comprises supplying a color former-containing ink to a color developer-coated sheet to obtain colored images (German Patent OLS No. 1,939,962).
The color developer has the above-defined properties and the examples thereof include acid clay, phenol resins, aromatic carboxylic acid metal salts, etc.
As the color former, there have been used those which are usually colorless, which possess a partial skeleton of lactone, lactam, sultone, spyropyran, ester, or amide, and which undergo ring cleavage or splitting of said partial skeleton when contacted with a color developer.
Specifically, there are illustrated triarylmethane compounds such as crystal violet lactone, xanthene compounds such as rhodamine B anilinolactam, spyropyran compounds such as 3-methyl-spiro-dinaphthopyran, thiazine compounds such as benzoyl leucomethylene bulue, and leucoauramine compounds such as N-(o-chlorophenyl)leucoauramine.
However, colored products formed by the reaction between a color former of triarylmethane compound, xanthene compound, spiro compound, or the like and an acid clay have poor durability, and, when in contact with water or an organic solvent, undergo reduction in density of color or disappearance of color. Of organic solvents, polar solvents such as alcohols (e.g., ethanol, butanol, propylene glycol, glycerin, etc.), amines (e.g., monoethanolamine, triethanolamine, etc.), and esters and ethers (e.g., ethyl acetate, diethylene glycol, monomethyl ether, dioctyl phthalate, dibutyl phthalate, etc.) exert strong influences on colored products when the organic solvents are contacted, directly or as a vapor, with the colored products, resulting in reduction in color density or the total disappearance of color.
The inventors have formerly proposed to use benzhydrylsulfone type color formers so as to overcome the above-described defects. However, recording sheets having coated thereon microcapsules retaining this benzhydrylsulfone type color former have the defect that the coated surface becomes bluish or that a cut section thereof undergoes color staining.
Further, when this microcapsule-coated paper is used in a so-called spot-printing process, which comprises printing a color developer-containing ink onto portions where copying (or recording) is necessary, some colored products are formed on the base paper surface other than color developer-coated portions (hereinafter referred to as base paper coloration). Thus, elimination of these defects has been eagerly desired.